Just to be clear, you're talking about finding a used bike before you get to the states, right? I'd expect good used bikes to have disappeared from the market by June, as everybody wants a cheap ride for the summer.

You might consider how much of a load you're going to carry before you decide. 15 pounds, no problem. 45 pounds of gear on a light/flimsy/racing bike is asking for shimmy, not to mention what it'll do to lightweight wheels. Bikes built specifically for touring are more likely to have a frame and wheels that are built for that kind of load.